Faults in electric power systems occur for a variety of reasons, such as trees falling across power lines, transformer failure, shorts in load circuits, and so forth. Once the line section experiencing the fault has been identified, sectionalizing switches are used to isolate the faulted line segment in order to bring the non-faulted sections back into service. Isolating the fault requires identification of the location of the fault with respect to a number of sectionalizing switches that are used to isolate faulted line sections. For a radial distribution circuit, the direction from the substation toward the load is referred to as the forward direction and the direction back toward the substation is referred to as the reverse direction. For a loop fed transmission or distribution circuit, however, the direction along a power line (forward or reverse) is defined as a matter of convention. Sectionalizing switches and associated monitoring equipment (e.g., voltage and current measuring devices) and fault controllers are typically located at the substations and at major tap points along the power line where transmission or distribution lines “T” from the main power line to pick up loads. When a fault occurs, one or two sectionalizing switches are typically operated to isolate the faulted line segment so that the non-faulted line segments can remain in service.
Determining the directionality of a fault on a three phase power line is conventionally accomplished with a voltage monitor and current monitor for each phase, requiring three voltage monitors and three current monitors at each monitoring station. Current monitors are relatively inexpensive whereas the voltage monitors can be a major expense. Although three phase voltage measurements are typically available in substations, they are not generally available at tap points along the power line. The extent of outages can be reduced, however, by locating directional fault detection equipment at the major tap points along the power line, not just at the substations. But this may not be economically feasible due to the high cost of installing three voltage monitors at each tap point.
There is, therefore, a continuing need for improved and more cost effective electric power fault isolation systems. There is, in particular, a need for a directional fault location and isolation system that can be used to operate sectionalizing switches at multiple tap points on three phase electric power lines.